Police at the scene of the attack in central London.
Photograph: Will Oliver/EPA

The Scotland Yard investigation into the Russell Square knife attack is expected to look at information held on the suspect by the health service. Based on initial police public statements, it is clear that any mental health records will be of particular and urgent interest.

Detectives will also see if the 19-year-old arrested was either known to the domestic security service MI5, or whether his name is associated with any known suspects. His phone and other electronic devices will be a key indicator of interest in extremist material, assuming they can be accessed. This will be crucial in making an early assessment of the suspect’s mindset and the presence or absence of a terrorist motive.

Witness testimony

The five survivors, when well enough, may have clues as to the reason for the attack. Detectives will want to know what, if anything, their attacker said before striking, as well as the person’s demeanour and whether any of them recognised or knew him.

The Russell Square area is rich with CCTV, and recordings will be seized on, with detectives seeking to piece together the attacker’s arrival in the area, the lead-up to the attack and whether others were involved. Footage that anyone shot on smartphones will also be sought.

Police did not appeal for information about anyone else, suggesting they believed there was only one attacker.

Terror link

Key questions the public and politicians will want answered are what makes the police think terrorism may be a possibility, and whether any extremist connection comes down to mental illness, ideology or a mixture of both.

If police, as well as their partners in MI5, decide the attack was driven by extremism, Scotland Yard’s counter-terrorism command would be expected to lead the investigation. Until then it would remain with the homicide command, supported by counter-terrorism specialists.

Mark Rowley – who spoke to the media in the early hours of Thursday after the Russell Square attack – is Britain’s most senior counter-terrorism officer. He serves as an assistant commissioner in the Metropolitan police and leads its specialist operations division, the home of the counter-terrorism command.

One way or another, the attacks in Russell Square represent a gap in Britain’s system of public safety – be it in national security, or in how Britain’s under-resourced and severely strained mental health system looks after those who need it most.

Mental health

Officials have said there is a crossover between terrorism and mental health problems. On Monday a man was jailed for life for a knife attack at an east London tube station in December 2015. A few hours after the attack police suggested it was probably terrorism. By the time that case came to court they accepted mental illness was more likely to be a significant factor.

In the December 2015 attack Muhiddin Mire shouted: “This is for my Syrian brothers. I’m going to spill your blood.” He also attacked or threatened four other travellers.

But the victim, the doctor who treated him and a substantial part of Britain’s security establishment said they believed Mire’s violence was a result of his acute mental health problems rather than a political motivation.

After further inquiries, Scotland Yard publicly said so – while pointing out that Mire had been inspired by Isis propaganda on his phone, the downloading of which appeared to coincide with his mental health deteriorating.

On Monday, despite these assessments, Mire was jailed for life with the judge saying he was motivated by Islamic extremism.

About half of all people feared to be at risk of terrorist sympathies may have mental health or psychological problems, a police study has found. The police study of 500 cases dealt with by Channel, an anti-radicalisation scheme, found that 44% of the individuals involved were assessed as being likely to have vulnerabilities related to mental health or psychological difficulties. A further 15% were assessed as possibly having vulnerabilities but more assessment was needed.

In May, Ch Const Simon Cole, who is in charge of the Prevent programme, which aims to protect people against radicalisation, told the Guardian: “There would appear to be, from the work we have been doing, a link to people who are vulnerable around mental health.”

The knife attack in central London on Wednesday evening came amid heightened fears of a terrorist attack in the UK.

Heightened fears

On Sunday, Britain’s most senior police officer, the Met commissioner Sir Bernard Hogan-Howe, said the likelihood of an attack was a matter of when, not if. On Wednesday, he announced that the number of firearms officers in the capital would be increased by 600. UK security officials have watched in alarm as France has suffered repeated attacks and other countries including Germany have been hit.

Since August 2014 the UK terrorist threat level has been raised to severe, its second-highest level. The government said the heightened alert was “related to developments in Syria and Iraq, where terrorist groups are planning attacks against the west”. That was a reference to Isis.

Terror attacks by Islamic extremists have twice claimed lives on Britain’s streets. The 2005 London attacks by four suicide bombers killed 52 people and left 750 injured, while Lee Rigby was butchered near the Woolwich military barracks in south London in May 2013 by Michael Adebolajo and Michael Adebowlale, who had spent just over £50 buying knives from an Argos store.